A flat-panel device contains a pair of generally flat plates connected together through an intermediate mechanism. The two plates are typically rectangular in shape. The thickness of the relatively flat structure formed by the two plates and the intermediate connecting mechanism is small compared to the diagonal length of either plate.
When used for displaying information, a flat-panel device is typically referred to as a flat-panel display. The two plates in a flat-panel display are commonly termed the faceplate (or frontplate) and the baseplate (or backplate). The faceplate, which provides the viewing surface, is part of a faceplate structure containing one or more layers formed over the faceplate. The baseplate is similarly part of a baseplate structure containing one or more layers formed over the baseplate. The faceplate structure and the baseplate structure are sealed together, typically through an outer wall.
A flat-panel display utilizes various mechanisms such as cathode rays (electrons), plasmas, and liquid crystals to display information on the faceplate. In a flat-panel cathode-ray tube ("CRT") display, electron-emissive elements are typically provided over the interior surface of the baseplate. When the electron-emissive elements are appropriately excited, they emit electrons that strike phosphors situated over the interior surface of the faceplate which consists of transparent material such as glass. The phosphors then emit light visible on the exterior surface of the faceplate. By appropriately controlling the electron flow, a suitable image is displayed on the faceplate.
Electron emission in a flat-panel CRT display needs to occur in a highly evacuated environment for the display to operate properly and to avoid rapid degradation in performance. The enclosure formed by the faceplate structure, the baseplate structure, and the outer wall is thus fabricated in such a manner as to be at a high vacuum, typically a pressure of 10.sup.-7 torr or less for a flat-panel CRT display of the field-emission type. Any degradation of the vacuum can lead to various problems such as non-uniform brightness of the display caused by contaminant gases that degrade the electron-emissive elements. The contaminant gases can, for example, come from the phosphors. Degradation of the electron-emissive elements also reduces the working life of the display. It is thus imperative that a flat-panel CRT display be hermetically sealed, that a high vacuum be provided in the hermetically sealed (airtight) enclosure, and that the high vacuum be maintained thereafter.
A field-emission flat-panel CRT display, commonly referred to as a field-emission display ("FED"), is conventionally sealed in air and then evacuated through tubulation provided on the display. FIG. 1 illustrates how one such conventional FED appears after the sealing and evacuation steps are completed. The FED in FIG. 1 is formed with baseplate structure 10, faceplate structure 11, outer wall 12, and multiple spacer walls 13. The FED is evacuated through pump-out tube 14, now closed, provided at opening 15 in baseplate structure 10.
Getter 16, typically consisting of barium, is commonly provided along the inside of tube 14 for collecting contaminant gases present in the sealed enclosure. This enables a high vacuum to be maintained in the FED during its lifetime. Getter 16 is of the evaporable (or flashable) type in that the barium is evaporatively deposited on the inside of tube 14.
Getter 16 typically performs in a satisfactory manner. However, tube 14 protrudes far out of the FED. Accordingly, the FED must be handled very carefully to avoid breaking getter-containing tube 14 and destroying the FED. It is thus desirable to eliminate tube 14. In so doing, the location for getter 16 along the inside of tube 14 is also eliminated.
Simply forming an evaporable barium getter at a location along the interior surface of baseplate structure 10 or/and faceplate structure 11 is unattractive. Specifically, a getter typically needs a substantial amount of surface area to perform the gas collection function. However, it is normally important that the active-to-overall area ratio--i.e., the ratio of active display area to the overall interior surface area of the baseplate (or faceplate) structure--be quite high in an FED. Because an evaporable barium getter is formed by evaporative deposition, a substantial amount of inactive area along the interior surface of the baseplate structure or/and the faceplate structure would normally have to be allocated for a barium getter, thereby significantly reducing the active-to-overall area ratio. In addition, the active components of the FED could easily become contaminated during the getter deposition process. Some of the active FED components could become short circuited.
A non-evaporable getter is an alternative to an evaporable getter. A non-evaporable getter typically consists of a pre-fabricated unit. As a result, the likelihood of damaging the components of an FED during the installation of a non-evaporable getter into the FED is considerably lower than with an evaporable getter. While a non-evaporable getter does require substantial surface area, the pre-fabricated nature of a non-evaporable getter generally allows it to be placed closer to the actual display elements than an evaporable getter.
Non-evaporable getters are manufactured in various geometries. FIGS. 2a and 2b (collectively "FIG. 2") illustrate the basic geometries for two conventional non-evaporable getters manufactured by SAES Getters. See Borghi, "St121 and St122 Porous Coating Getters," SAES Getters, Jul. 27, 1994, pages 1-13. The getter in FIG. 2a consists of metal wire 18A covered by coating 19A of gettering material. The getter in FIG. 2b consists of metal strip 18B covered by coating 19B of gettering material. A porous mixture of titanium and a zirconium-containing alloy typically forms the gettering material in these two non-evaporable getters.
Upon being placed in a highly evacuated environment, each of the getters in FIG. 2 is activated by raising the temperature of getter coating 19A or 19B to a suitably high value, typically 500.degree. C., for a suitably long activation time, typically 10 min. At constant activation time, the getter performance can be increased by raising the activation temperature. For the getters of FIG. 2, the activation temperature can be as high as 900-950.degree. C. above which the getters may be permanently damaged. Alternatively, as the activation temperature is increased, equivalent performance can be achieved at reduced activation time. The opposite occurs as the activation temperature is lowered to as little as 350.degree. C. below which the gettering performance of the getters in FIG. 2 is significantly curtailed.
A getter typically consists of a porous mixture of particles that sorb gases which contact the outer surfaces of the particles. When the non-evaporable getters of FIG. 2 are activated in a high vacuum environment, sorbed gases present on the outer surfaces of the getter particles diffuse into the bulk of the getter particles, leaving their outer surfaces free to sorb more gases. The amount of gas which can be accumulated in the bulk of getter particles that are accessible to the gases is typically much more than the maximum amount of gas that the getter can sorb on the outer surfaces of the accessible particles. When the accessible outer getter surface is filled or partially filled with sorbed gases, the getter can be re-activated in a high vacuum environment to transfer the gases on the accessible outer surface to the bulk of the getter particles and again leave the accessible outer surface free to sorb more gases. Re-activation can typically be performed a relatively large number of times.
Borghi mentions three ways of activating the getters of FIG. 2 under high vacuum conditions: (a) resistive heating, (b) RF heating, and (c) indirect heating. Resistive heating is performed by passing current through metallic conductor 18A or 18B to raise the temperature of getter coating 19A or 19B to the activation temperature. The current and accompanying power are relatively high during the activation process, facts that must be taken into account in utilizing resistive heating to activate the getters. Borghi also mentions that the getters can be activated during bake-out treatments of the vacuum devices that contain the getters.
Wallace et al, U.S. Pat. No. 5,453,659, discloses a getter arrangement for an FED in which the gettering material is distributed across the active area of the faceplate structure. As shown in FIG. 3.1, the faceplate structure in Wallace et al contains transparent substrate 20, thin electrically insulating layer 21, electrically conductive anode regions 22, and phosphor regions 23. Electrically insulating material 24 of greater thickness than anode regions 22 is situated in the spaces between regions 22. Gettering material 25 is situated on insulating material 24 and is spaced apart from phosphor regions 23. Wallace et al indicates that getter material 25 can be barium or a zirconium--vanadium--iron alloy.
Getter material 25 in Wallace is initially activated during assembly of the FED under high vacuum conditions at 300.degree. C. Wallace et al also provides circuitry, including electrical conductors connected to getter material 25, for re-activating getter material 25.
The getter arrangement of Wallace et al appears relatively efficient in terms of area usage. However, getter material 25 is relatively complex in shape and requires manufacturing steps that could be unduly expensive. The necessity to maintain space between getter material 25 and phosphor regions 23 raises reliability concerns. The provision of circuitry to re-activate getter material 25 raises further reliability concerns and also further increases the fabrication cost. It would be desirable to have a simple technique for activating/re-activating a getter, especially one of relatively simple design, in a flat-panel device without raising the reliability concerns of Wallace et al, without incurring high getter installation costs, and without using an awkward getter-containing attachment such as the pump-out tubulation commonly used with evaporable getters in FEDs.
Pepi, U.S. Pat. No. 5,519,284, discloses a composite getter/pump-out arrangement that overcomes much of the awkwardness present in the conventional getter/pump-out arrangement of FIG. 1. FIG. 3.2a shows Pepi's getter/pump-out arrangement in which plate 25 of a flat display screen, such as an FED, has pump-out aperture 26. Pump-out tube 27 overlies aperture 26 and is bonded to the exterior surface of plate 25. Pump-out tube 27 has constricted portion 27A which broadens into circular cylindrical portion 27B having concave wall 27C. A group of getters 28 lie on the exterior surface of plate 25 below concave wall 27C. Pepi specifies that getters 28 may consist of cylindrical bars or strips. Pepi also discloses that the gettering material may be evaporatively deposited onto broadened tube portion 27B.
Pepi's flat display screen is pumped out through tube 27. Subsequently, tube 27 is closed at constricted portion 27A as shown in FIG. 3.2b. The closure operation is performed in such a way that the remainder 27D of constricted portion 27A lies below the highest part of broadened tube portion 27B.
Pepi's getter/pump-out arrangement enables getters 28 to be located in a pump-out tube which, after tube closure, does not protrude far from the flat display screen. This should reduce the likelihood of damaging the display compared to the getter/pump-out arrangement of FIG. 1. However, closing tube 27 appears to involve heating constricted portion 27A along a location very close to concave wall 27C. Undesired stresses may be produced in concave portion 27C, thereby forming a weak point in the display. Also, when getter material is evaporatively deposited onto broadened tube portion 27B (including concave wall 27C), some of the evaporated getter material may pass through pump-out aperture 26 and contaminate the active display elements. It would be desirable to have a simple FED getter arrangement that overcomes the disadvantages of Pepi's arrangement and is suitable for a non-evaporable getter.
FIG. 3.3 illustrates the FED of Wiemann et al, U.S. Pat. No. 5,545,946, in which gated electron emitters 30 are provided in substrate 31 situated between backplane 32 and a faceplate structure consisting of faceplate 33, anode layer 34, and cathodoluminescent material layer 35. Electrons emitted from gated emitters 30 enter substrate apertures 31A and then move through interspace apertures 36A in electrically insulating layer 36 to strike cathodoluminescent material 35. Spacers 37 maintain a fixed spacing between electron emitters 30 and thin gettering layer 38 overlying backplane 32. Getter 38, which appears to be maintained at negative potential relative to anode layer 34, collects contaminant gases present in apertures 36A and 31A and the evacuated region between substrate 31 and getter 38.
By having gettering layer 38 situated on a different level than emitter-containing substrate 30 or the faceplate structure, the FED of Wiemann et al achieves a high active-to-overall area ratio. This is advantageous. However, it is not clear how getter 38 is activated or whether it can be reactivated. Furthermore, the presence of getter 38 and accompanying spacers 37 causes the overall thickness of the FED to be significantly increased, an undesirable result. In an FED containing a getter, it would be desirable to achieve a high active-to-overall area ratio without having the presence of the getter cause a significant increase in the overall FED thickness.